1. Field of the Invention
The present invention relates to an organic electroluminescent device or element (hereinafter, referred also to as an xe2x80x9corganic EL devicexe2x80x9d) which is utilized as a planar light source or utilized in display devices.
2. Description of the Related Art
Attention has been made to an organic EL device in which a luminescent layer, i.e., light emission layer is formed from a specific organic compound, because it ensures a large area display device which can be operated at a low voltage. To obtain a highly efficient EL device, Tang et al., as is reported in Appl. Phys. Lett., 51, 913 (1987), have succeeded in attaining an EL device having a structure in which organic compound layers having different carrier transporting properties are laminated to thereby introduce holes and electrons with a good balance from an anode, and a cathode. In addition, since the thickness of the organic compound layers is not more than about 2,000 xc3x85, the EL device can exhibit a high luminance and efficiency sufficient for practical use; i.e., a luminance of about 1,000 cd/m2 and an external quantum efficiency of about 1% at an applied voltage of not more than about 10 volts.
In this highly efficient EL device, Tang et al. have used magnesium (Mg) having a low work function in combination with an organic compound which is essentially considered to be an electrically insulating material, in order to reduce an energy barrier which can cause a problem during injection of electrons from a metal-made electrode. However, since the magnesium is liable to be oxidized and is unstable, and also exhibits only a poor adhesion to a surface of the organic layers, magnesium was used after alloying. Alloying is carried out by vapor co-deposition or simultaneous vapor evaporation of magnesium and silver (Ag) which is relatively stable and exhibits good adhesion to a surface of the organic layers.
Further, in the EL device developed by Tang et al., an indium-tin-oxide (ITO) is coated as an anode electrode over a glass substrate. However, the use of the ITO anode electrode device in the Tang et al. to obtain good contact (near to ohmic contact) is considered to be made due to an unexpected and fortunate occurrence; namely, the ITO electrode is frequently used as a transparent anode electrode made of metal oxide in the hole injection of the organic compound to satisfy the requirement for the omission of light in the planar area, and the ITO electrode can exhibit a relatively large work function of a maximum of 5.0 eV.
Furthermore, in their EL device, Tang et al. have inserted a layer of copper phthalocyanine (hereinafter termed as xe2x80x98CuPcxe2x80x99) having a thickness of not more than 200 xc3x85 between the anode and the hole-transporting organic compound layer to further improve contact efficiency of the anode interface region, thereby achieving the operation of the device at a low voltage.
Similar effects have been also confirmed from the starburst type arylamine compounds, proposed by Shirota et al. of Osaka University, by the researchers of Pioneer Co., Ltd. Both the CuPc compounds and the starburst type arylamine compounds have characteristics that show a work function smaller than that of ITO, and a relatively high mobility of hole charge, and thus improving the stability of the EL devices during the continuous usage thereof, facilitating low-voltage consumption and an improved interfacial contact.
In addition to the above-described devices having the vacuum evaporated layers, there are also known EL devices having the layers formed from a coating solution of a film-forming polymeric material by a coating method such as spin coating. In such EL devices, the coating solution is prepared by previously dispersing an electron-accepting compound in a hole-transporting polymeric material. For example, Partridge, as is reported in POLYMER, Vol.24, June 1983, has confirmed that an ohmic current can be obtained if an antimony pentachloride (hereinafter, SbCl5) as an electron-accepting compound is dispersed in dichroromethane solution of polyvinyl carbazole (hereinafter, PVK), whereas such ohmic current could not been realized with the sole use of PVK in the layer formation. In this layer formation, it is understood that SbCl5 can act as a Lewis acid so that a carbazole pendant group of PVK is oxidized to produce radical cations. SbCl5 used by Partridge in the layer formation is in a liquid state at a room temperature, and is a Lewis acid compound having a remarkably high reactivity so that fumes can be produced upon reaction with water in atmospheric air. However, contrary to this, if it is reacted with PVK in a glove chamber under an inert atmosphere, SbCl5 can form a stable complex compound, thereby enabling to form a layer of the complex compound under relatively stable conditions of atmospheric air. Thus, the above layer formation method is considered to be a rational one if it is intended to form a hole injection layer from a side of the ITO electrode. However, in the recent organic EL devices, a highly increased efficiency of the device has been achieved largely relying upon the high purity layer formation process which is based on vacuum evaporation and does not cause cross-contamination. In this production of EL devices, assuming that the above-described method by Partridge is directly applied without any modification, some questions arise because stable driving of the EL devices can be adversely affected by any residues of the solvent used in the coating solution and any impurities in the layer-forming materials.
The present invention is directed to solve the above-described problems in the EL devices of the prior art. An object of the present invention is to reduce the energy barrier in the hole injection from a transparent ITO anode electrode to a hole-transporting organic layer, and to achieve low-voltage consumption regardless of the work function of the anode material.
To accomplish the above object, the inventors have researched extensively and have now discovered that in the hole injection from an anode electrode to an organic layer adjacent to the anode electrode, an injection barrier (and thus, the voltage) can be reduced if the organic layer is doped with a compound capable of acting as an electron-accepting dopant by a co-deposition or simultaneous evaporation method.
According to the present invention, there is provided an organic electroluminescent (EL) device including at least one light emission layer from an organic compound, the light emission layer being positioned between an anode electrode and a cathode electrode opposed to the anode electrode, in which an organic layer positioned adjacent to the anode electrode is from an organic compound and includes, as an electron-accepting dopant, an electron-accepting compound having a property of oxidizing the organic compound of said organic layer, said electron-accepting compound being doped to said organic layer in vacuum by a simultaneous evaporation method.
In the organic EL devices, the hole injection process from an anode to an organic layer which is basically constituted from an electrically insulating organic compound is intended to carry out oxidation of the organic compound on a surface region of the organic layer, i.e., formation of a radical cation state thereof (cf., Phys. Rev. Lett., 14, 229 (1965)). In the organic EL device, an electron-accepting dopant substance or compound which can act as an oxidizing agent for the organic compound is previously doped in an organic layer in contact with the anode electrode, and thus the energy barrier of the hole injection from the anode electrode can be lowered as a result of such provisional doping of the dopant compound in the organic layer. Since the molecules in the oxidized state (oxidized by the dopant), i.e., in the electron-donated state, are already included in the doped organic layer, a barrier of the hole injection energy is low in the EL device, and therefore a driving voltage of the device can be lowered in comparison with the EL devices of the prior art.
In practice, the electron-accepting dopant used in the formation of the organic layer in contact with the anode electrode may be either an inorganic compound or an organic compound, as long as they have an electron-accepting property and can oxidize an organic compound in the organic layer. Particularly, suitable electron-accepting dopant compounds in the form of an inorganic compound include a Lewis acid compound such as ferric chloride, aluminum chloride, gallium chloride, indium chloride, antimony pentachloride. Further, if an organic dopant compound is used, suitable dopant compounds have a property of oxidizing an organic compound used in the hole injection layer through Lewis acid chemistry, such as trinitrofluorenone. These dopant compounds may be used alone or in combination.
When the above-described dopant compounds are doped in the organic layer by the co-deposition method, the compounds having a relatively low saturated vapor pressure such as ferric chloride and indium chloride can be contained in a crucible, followed by depositing using a conventional resistance heating method. Alternatively, if the dopant compounds used have a high vapor pressure at an ordinary temperature and therefore the pressure in the vacuum deposition apparatus can not be maintained at a level below the predetermined degree of vacuum, the vapor pressure may be controlled by using an orifice (opening size)-controlling means such as a needle valve or mass flow controller or by using a susceptor or sample-supporting means having a separate temperature-controlling system to cool the dopant compounds.
A thickness of the produced doped organic layer is not restricted to the specific thickness range, however, it is generally preferred that the thickness is not less than 5 xc3x85. In the organic layer, the organic compounds contained therein can be present in a state of radical cations in the absence of electric field, and therefore they can act as an internal charge. Namely, no specific requirement of the layer thickness is given to the organic layer, and therefore a layer thickness of the organic layer can be increased without causing an undesirable increase of the voltage of the device. Therefore, in the EL device, the organic layer can be also utilized as a means for remarkably diminishing the possibility of short-circuiting, if a distance between the opposed electrodes of the device has increased more than conventional EL devices. Accordingly, it becomes possible to increase a total thickness of the organic layer(s) between the electrodes to 2,000 xc3x85 or more.
In the doped organic layer, the concentration of the dopant compound is not restricted to a specific range; however, it is generally preferred that a molar ratio of the organic compound or molecule to the dopant compound or molecule (i.e., organic molecule : dopant molecule) is in the range of about 1:0.1 to about 1:10. The molar ratio of the dopant molecule of less than 0.1 will result in only a poor doping effect, because a concentration of the molecules oxidized with the dopant (hereinafter xe2x80x9coxidized moleculesxe2x80x9d) is an excessively low level. Similarly, if the molar ratio of the dopant molecule is above 10 times, only a poor and reduced doping effect will be obtained, because the dopant concentration is remarkably increased beyond that of the organic molecule, thereby causing an excessive reduction of the concentration of the oxidized molecules in the organic layer.
The present disclosure relates to subject matter contained in Japanese Patent Application No.10-49771 (filed on Mar. 2, 1998) which is expressly incorporated herein by reference in its entirety.